In general, most people like to have their windows cleaned twice a year, once in late spring after the rainy season, and once in late fall. It's not adviseable to let more than a year go by between cleanings; the elements are harsh, sediment will build up and the sun will bake it on, causing pitting and hard water deposits among other problems.

Some people get their windows cleaned every three to four months (people who do a lot of entertaining, office buildings). Some people get their windows cleaned one or more times per month (storefronts). It's really up to you and your needs, but again, we usually recommend twice yearly cleanings.